Plasma etching techniques

ABSTRACT

In certain embodiments, a method of processing a semiconductor substrate includes positioning a semiconductor substrate in a plasma chamber of a plasma tool. The semiconductor substrate includes a film stack that includes silicon layers and germanium-containing layers in an alternating stacked arrangement, with at least two silicon layers and at least two germanium-containing layers. The method includes exposing, in a first plasma step executed in the plasma chamber, the film stack to a first plasma. The first plasma is generated from first gases that include nitrogen gas, hydrogen gas, and fluorine gas. The method includes exposing, in a second plasma step executed in the plasma chamber, the film stack to a second plasma. The second plasma is generated from second gases comprising fluorine gas and oxygen gas. The second plasma selectively etches the silicon layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/155,772, entitled “Plasma Etching Techniques,” filed on Jan. 22,2021, now issued as U.S. Pat. No. 11,424,120, which application isincorporated herein by reference.

TECHNICAL FIELD

This disclosure relates generally to semiconductor fabrication, and, incertain embodiments, to plasma etching techniques.

BACKGROUND

The integrated circuit (IC) manufacturing industry strives to increasedevice density to improve speed, performance, and costs. For continuedscaling to smaller node sizes, device architectures have evolved fromtwo-dimensional (2D) planar structures to three-dimensional (3D)vertical structures, such as with nanowires or vertically orientedtransistors. Insufficient control of the conducting channel by the gatepotential drives a desire for this change. Short channel effects (SCE)may become too significant as gate dimensions are scaled down and mayincrease current conduction when no voltage is applied to the gate(I_(off)). A change in device architecture may allow betterelectrostatic control of the gate to reduce the SCE and power loss.Fabricating nanowire devices may present 3D etch challenges where highlyselective isotropic etch processes are beneficial. For example, layersof exposed materials may need to be etched relative to one another tocreate indents in a film stack.

SUMMARY

In certain embodiments, a method of processing a semiconductor substrateincludes receiving a semiconductor substrate that includes a film stack.The film stack includes a first germanium (Ge)-containing layer, asecond Ge-containing layer, and a first silicon (Si) layer positionedbetween the first Ge-containing layer and the second Ge-containinglayer. The method includes modifying, in a first plasma step, exposedsurfaces of the first Ge-containing layer, the second Ge-containinglayer, and the first Si layer by exposing the exposed surfaces to afirst plasma. Modifying the exposed surfaces includes removing at leasta portion of a native oxide layer (NOL) from the exposed surfaces of thefirst Si layer and forming a passivation layer on the exposed surfacesof the first Ge-containing layer and the second Ge-containing layer. Themethod includes, in a second plasma step, etching, using a secondplasma, the first Si layer to form an indent in the film stack at thefirst Si layer between the first Ge-containing layer and the secondGe-containing layer. The passivation layer inhibits etching of the firstGe-containing layer and the second Ge-containing layer.

In certain embodiments, a method of processing a semiconductor substrateincludes positioning a semiconductor substrate in a plasma chamber of aplasma tool. The semiconductor substrate includes a film stack thatincludes Si layers and Ge-containing layers in an alternating stackedarrangement, with at least two Si layers and at least two Ge-containinglayers. The method includes exposing, in a first plasma step executed inthe plasma chamber, the film stack to a first plasma. The first plasmais generated from first gases that include nitrogen gas, hydrogen gas,and fluorine gas. The method includes exposing, in a second plasma stepexecuted in the plasma chamber, the film stack to a second plasma. Thesecond plasma is generated from second gases comprising fluorine gas andoxygen gas. The second plasma selectively etches the Si layers.

In certain embodiments, a method of processing a semiconductor substrateincludes positioning a semiconductor substrate in a plasma chamber of aplasma tool. The semiconductor substrate includes a film stack thatincludes Si layers and Ge-containing layers in an alternating stackedarrangement, with at least two Si layers and at least two Ge-containinglayers. The method includes generating a plasma in the plasma chamber.Generating the plasma includes injecting, into the plasma chamber, gasesthat include an etchant-containing gas, a passivation-triggering gas,and a carrier gas. The plasma includes etchant agents and passivationagents. The method includes exposing the film stack to the plasma in theplasma chamber. The method includes terminating, after a time period,injecting the etchant-containing gas into the plasma chamber andcontinuing injecting the passivation-triggering gas into the plasmachamber.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, and advantagesthereof, reference is made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1A-1F illustrate cross-sectional views of an example semiconductorsubstrate during an example process for processing the semiconductorsubstrate;

FIGS. 2A-2C illustrate cross-sectional views of an example semiconductorsubstrate during example process for processing the semiconductorsubstrate;

FIG. 3 illustrates an example timeline of gas injections andterminations;

FIG. 4 illustrates an example method for processing a semiconductorsubstrate;

FIG. 5 illustrates an example method for processing a semiconductorsubstrate;

FIG. 6 illustrates an example method for processing a semiconductorsubstrate;

FIG. 7 illustrates an example device including a substrate with arecessed alternating film stack; and

FIG. 8 illustrates a block diagram of an example plasma tool.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Various techniques for attempting to selectively etch one materialrelative to another exist. In some cases the chemistry of two materialsis sufficiently distinct to allow a plasma that is selective to etchingone of the materials to be used without concern for etching the othermaterial. In other cases determining appropriate etching regimes forselective etching is more difficult because the chemistry of thematerials may be similar or the available etching processes may belimited by other factors. Certain materials present more difficultselectivity challenges where it is desirable to etch one material withlittle to no etching of another material. Conventional processes forthis type of selective etching may be unable to achieve selectiveetching of one material relative to another or may fall short of processrequirements such as selectivity, etch profile (e.g., local uniformityand/or surface roughness), and others.

Selectivity challenges may arise in forming nanowires or nanosheets toact as a channel region in a 3D vertical structure of a semiconductordevice, such as a gate-all-around (GAA) device. Forming such nanowiresmay involve forming a film stack on a base layer, the film stackincluding layers of Si and Ge or Si—Ge (SiGe) arranged in an alternatingstack. Part of this process may include etching indents, or recesses, inthe film stack at opposing ends of the Si layers, while minimizingetching of the Ge-containing layers, to expose end portions of theGe-containing layers for later use as a conducting device. Due tovarious challenges, including in part a NOL and/or other residues (e.g.reactive ion etching residue) present on surfaces of the film stack,conventional etching techniques may be unsatisfactory.

For example, some conventional techniques employ a single etch stepusing a plasma generated from nitrogen trifluoride (NF₃) (or anotheretchant) and oxygen (O₂), without any prior step to remove the NOL.Fluorine radicals in this plasma may etch the Si layers, while oxygenmay react with the Ge in the Ge-containing layers to form a Ge oxide(e.g., GeO₂) protection layer on the Ge-containing layers. Post-etch,however, resulting structures of such a single etch step without anyprior step to remove or otherwise modify the NOL and/or without anycontinuing formation of a passivation layer at an end portion of theetch step generally show unacceptable levels of surface roughness alongexposed surfaces of the Si layers and the Ge-containing layers,resulting in part from etching the NOL, and gouging into Ge-containinglayers near the etch front of the Si layers due to the shorter lifetimeof passivation agents (e.g., oxygen) as opposed to etchants (e.g.,fluorine).

As another example, prior to performing the plasma etch to form indentsin the film stack, some conventional techniques use a wet or dry processto remove the NOL. The film stack may be processed using a dilutehydrogen-fluoride (HF) acid or a chemical oxide removal process.Removing the NOL over surfaces of both the Si layers and Ge-containinglayers, however, may reduce the selectivity of the subsequent indentplasma etch process (e.g., using NF₃ and O₂) to etch the Si layersrelative to the Ge-containing layers. Also, these native oxide removalprocesses are performed in a tool separate from the tool used to performthe indent plasma etch, adding time and cost to the integratedfabrication process.

As another example, in the etch step using a plasma generated from NF₃(or another etchant) and oxygen (O₂), when the time to terminate theplasma etch process occurs, conventional techniques terminate the flowof both the etchant-containing gas (e.g., NF₃) and thepassivation-triggering gas (e.g., O₂) (that is, the gas that results inthe presence of passivation reactants in the generated plasma that causea passivation layer to be formed on exposed surfaces of theGe-containing layers) at substantially the same time. However, theetchants (e.g., fluorine atoms) in the plasma have longer lifetimes thanthe passivation reactants (e.g., oxygen atoms) in the plasma, such thatthe passivation reactants dissipate more quickly than the etchants. Inone example (though particular values may depend on a variety offactors), oxygen passivation reactants expire in a few tenths of asecond, while fluorine etchants expire in about 2 to about 5 seconds orpossibly more. This property means the film stack will continue to beexposed to the etchants for a time period, with little to no passivationreactants present to form a passivation, or protection, layer on exposedsurfaces of the Ge-containing layers, undesirably allowing the etchantsto etch portions of the Ge-containing layers. An immediate purge of theplasma chamber of a plasma tool is insufficient to address this problem,at least because the relatively lengthy mechanical purge process doesnot remove the etchants fast enough to account for the differentlifetimes of the passivation reactants and the etchants.

The longer etchant lifetime may result in reduced or completely lostselectivity of the plasma to etch the Si layers rather than theGe-containing layers. As a result, so-called “gouging” of theGe-containing layers may occur at the etch front in areas near theinterface of the Ge-containing layers and their adjacent Si layers inthe alternating film stack, as well as excessive etching of the Silayers. The gouging may be particularly present at the etch front (areasof Ge-containing layers most recently exposed to the etchant), as thoseareas of the Ge-containing layers have experienced reduced or noexposure to the passivation reactants. Furthermore, this gouging of theGe-containing layers at the etch front may cause the Ge-containinglayers to have poor local uniformity, with an undesirable thicknessloss, including at areas near the interface of the Ge-containing layersand their adjacent Si layers in the film stack.

Embodiments described below provide various methods of selectiveetching. For example, embodiments may be used to selectively etchportions of a film stack (e.g., that includes Si layers andGe-containing layers in an alternating stacked arrangement) of asubstrate. It may be desirable to selectively etch indents, or recesses,in edge portions of (or possibly completely remove) the Si layers toform nanowires of the Ge-containing layers.

Certain embodiments use a two-step plasma process to form indents in afilm stack that includes Si layers and Ge-containing layers in analternating stacked arrangement. In a pre-etch first step (e.g., asurface modification step), a substrate that includes the film stack isexposed to a first plasma that may substantially remove or otherwisechemically alter a barrier layer (e.g., an NOL) over exposed surfaces ofthe Si layers and cause a passivation layer (e.g., a Ge-nitride or Geoxynitride passivation layer) to be formed on exposed surfaces of theGe-containing layers. In a second step, the substrate, as modified inthe surface modification step, is exposed to a second plasma toselectively etch edge portions of the Si layers, the passivation layerinhibiting etching of the Ge-containing layers during exposure to thesecond plasma. The two-step plasma process may be performed in situ in aplasma chamber of a plasma tool.

Certain embodiments terminate injection (flow) of the etchant-containinggas (e.g., NF₃) into the plasma chamber prior to terminating injection(flow) of the passivation-triggering gas (e.g., O₂), allowingpassivation reactants (e.g., oxygen atoms) in the generated plasma tocontinue to form a passivation layer on exposed surfaces of theGe-containing layers while the etchants (e.g., fluorine atoms) in thegenerated plasma expire.

FIGS. 1A-1F illustrate cross-sectional views of an example substrate 102during an example process 100 for processing substrate 102, according tocertain embodiments. Process 100 may include a pre-etch first plasmaprocess to modify certain surfaces of substrate 102 and a second plasmaprocess to etch portions of certain layers of a film stack of substrate102, resulting in substrate 102 having an indented film stack followingexecution of process 100.

As shown in FIG. 1A, substrate 102 is a semiconductor substrate thatincludes film stack 104 disposed on a base layer 106. Film stack 104includes Si layers 108 and Ge-containing layers 110 in an alternatingstacked arrangement. Film stack 104 may have any suitable shape andinclude any suitable number of layers. As examples, the verticalthickness of Si layers 108 and Ge-containing layers 110 may be about 10nm to about 25 nm each, and as particular examples may be about 10 nm orabout 20 nm. Si layers 108 may have the same thicknesses or vary inthickness relative to one another, Ge-containing layers 110 may have thesame thickness or vary in thickness relative to one another, and Silayers 108 and Ge-containing layers 110 may have the same thicknesses orvary in thickness relative to one another. In an example, Si layers 108and Ge-containing layers 110 all have substantially the samethicknesses.

The material of Si layers 108 may be pure Si or Si nitride (SiN), forexample. In certain embodiments, all Si layers 108 include the samematerial; however, Si layers 108 may include different materials ifdesired.

The material of Ge-containing layers 110 may be pure Ge or SiGe alloy,for example. As a particular example, the Ge-containing layers 110 mayinclude a SiGe alloy (mixture) in an appropriate ratio (e.g.,Si_(0.7)Ge_(0.3), Si_(0.75)Ge_(0.25), etc.) for desired etchingproperties of a given application or for desired performance in aresulting semiconductor device formed using, in part, process 100. Incertain embodiments, all Ge-containing layers 110 include the samematerials; however, Ge-containing layers 110 may include differentmaterials if desired.

Base layer 106 may be any suitable material and includes Ge or SiGealloy in one example. In a particular example, film stack 104 is formedby growing alternating heteroepitaxial layers of Si (e.g., Si layers108) and Ge or SiGe (e.g., Ge-containing layers 110) atop base layer106.

An optional hard mask 112 may be included on top of film stack 104. Hardmask 112 may have been used to form the structure of film stack 104, ina previous etch process for example. In certain embodiments, hard mask112 is SiN but may include any suitable material.

A barrier layer 114 is formed over film stack 104 (including hard mask112) and, in this example, base layer 106. Barrier layer 114 may resultfrom prior fabrication steps (e.g. reactive ion etching) applied tosubstrate 102 or to other handling of substrate 102. As particularexamples, barrier layer 114 may include an NOL, reactive ion etchingresidue, or both. An NOL may be a thin layer of SiO₂ (or other suitablematerial), about 1 nm to about 2.0 nm thick for example, that forms onsurfaces of substrate 102, such as when substrate 102 is exposed toambient air, which contains O₂ and H₂O, when transferring betweenprocessing tools. For example, surfaces of base layer 106, Si layers108, Ge-containing layers 110, and hard mask 112 may interact with theambient air, which may result in barrier layer 114 at those surfaces. Asanother example, surfaces of base layer 106, Si layers 108,Ge-containing layers 110, and hard mask 112 may include a residueresulting from prior reactive ion etching steps.

Barrier layer 114 may have different etch properties than layers thatunderlie barrier layer 114. Although shown as having generally uniformcoverage over film stack 104 (including hard mask 112) and base layer106, barrier layer 114 might or might not have uniform coverage.

Each of the layers in film stack 104 has a pair of exposed surfaces atopposed ends when viewed, as illustrated, from a cross-sectionalperspective. That is, each of Si layers 108 has (opposing) exposedsurfaces 116, and each of Ge-containing layers 110 has (opposing)exposed surfaces 118. Additionally, in the state illustrated in FIG. 1A,because substrate 102 includes barrier layer 114, exposed surfaces of116 of Si layers 108 and exposed surfaces 118 of Ge-containing layers110 include barrier layer 114.

As illustrated in FIG. 1B, in a plasma step 120 of process 100,substrate 102 is exposed to plasma 122 to modify exposed surfaces 116 ofSi layers 108 and exposed surfaces 118 of Ge-containing layers 110.Plasma step 120 also may be referred to as a surface modification step.Plasma step 120 may be performed in a plasma chamber 123 of a plasmatool. The plasma tool may be any suitable type of plasma tool, includinga capacitively-coupled plasma (CCP) tool, an inductively-coupled plasma(ICP) tool, a surface wave plasma (SWP) tool, an electron cyclotronresonance (ECR) plasma tool, and others. An example plasma tool isdescribed below with regard to FIG. 8 .

In certain embodiments, modifying exposed surfaces 116 of Si layers 108includes removing all or a portion of barrier layer 114 from exposedsurfaces 116. For example, modifying exposed surfaces of 116 of Silayers 108 may include removing or reducing a thickness of barrier layer114 from exposed surfaces 116. Additionally or alternatively, modifyingexposed surfaces 116 of Si layers 108 may include chemicallytransforming barrier layer 114 at exposed surfaces 116 into a form thatis more easily etched in a subsequent etch step (e.g., the etch stepdescribed below with reference to FIG. 1D).

In certain embodiments, modifying exposed surfaces 118 of Ge-containinglayers 110 includes forming a passivation layer 124 on exposed surfaces118. Plasma 122 may cause passivation layer 124 to form on exposedsurfaces 118 of Ge-containing layers no by removing and replacing orotherwise modifying portions of barrier layer 114 on exposed surfaces118. For example, forming passivation layer 124 may include removingbarrier layer 114 from (or reducing the thickness of barrier layer 114on) exposed surfaces 118 of Ge-containing layers 110 and modifyingresulting exposed surfaces 118 to include passivation layer 124.Passivation layer 124 also may be formed on exposed surfaces of baselayer 106, such as when base layer 106 is pure Ge or includes Ge (e.g.,a SiGe base layer 106).

Although passivation layer 124 may have any suitable thickness, incertain embodiments, passivation layer is relatively thin, such as 2 nmor less. Passivation layer 124 could be, for example, a monolayer. Incertain embodiments, passivation layer 124 includes nitrogen, such as Genitride (e.g., Ge₃N₄), or Ge oxynitride (GeON).

Thus, in contrast to certain conventional techniques that might stripbarrier layer 114 using a dry or wet etch process both from exposedsurfaces 116 of Si layers 108 and from exposed surfaces 118 ofGe-containing layers no, leaving exposed surfaces 118 of Ge-containinglayers 110 without a protective etch stop layer and thereby sacrificingselectivity to Ge-containing layers 110 in a subsequent etch step (e.g.,the etch step described below with reference to FIG. 1D), plasma step120, using plasma 122, modifies (and possibly remove) barrier layer 114at exposed surfaces 116 of Si layers 108 while also forming passivationlayer 124 at exposed surfaces 118 of Ge-containing layers 110.Furthermore, in contrast to conventional techniques that might stripbarrier layer 114 using a dry or wet etch process, plasma step 120 maybe performed in a same plasma tool that is used to perform a subsequentetch step (e.g., the etch step described below with reference to FIG.1D).

Plasma 122 may include fluorine agents 126 (e.g., atomic fluorine),hydrogen agents 128 (e.g., atomic hydrogen), and nitrogen agents 130(e.g., atomic nitrogen). Fluorine agents 126 may act as the etchant forremoving or otherwise modifying some or all of barrier layer 114.Hydrogen agents 128 may act as a reducing agent, facilitating thebreakdown of barrier layer 114 in the presence of fluorine agents 126.Furthermore, if applicable depending on the gases used to generateplasma 122, hydrogen agents 128 may further break down certain compoundsof fluorine and nitrogen to produce fluorine agents 126 and nitrogenagents 130. Nitrogen agents 130 in plasma 122 react with the Ge atexposed surfaces 118 of Ge-containing layers 110 to form a nitride(e.g., a Ge nitride, Ge₃N₄, or Ge oxynitride, GeON) passivation layer124 at exposed surfaces 118. For example, the atomic nitrogen (N)generated in plasma 122 may react with the Ge molecules at exposedsurfaces 118 of Ge-containing layers 110 (and exposed surfaces of baselayer 106) to form a passivation layer 124 (e.g., a nitride layer) onexposed surfaces 116 of Ge-containing layers 110 (and on exposedsurfaces of base layer 106).

Plasma 122, containing fluorine agents 126, hydrogen agents 128, andnitrogen agents 130, when processed under appropriate conditions inplasma chamber 123, may generate chemically reactive neutrals such as N,H, NH, NH₂, F, F₂, HF, and/or NH₃, which interact with barrier layer 114at exposed surfaces 116 of Si layers 108 and exposed surfaces 118 ofGe-containing layers 110. This interaction may reduce or remove barrierlayer 114 at exposed surfaces 116 of Si layers 108 and exposed surfaces118 of Ge-containing layers 110. For example, this interaction maychemically modify barrier layer 114 at exposed surfaces 116 of Si layers108 to be more easily etched in a subsequent etch step. As anotherexample, this interaction may modify barrier layer 114 at exposedsurfaces 116 of Si layers 108 by thinning or completely removing barrierlayer 114 at exposed surfaces 116. Furthermore, this interaction mayreduce or remove barrier layer 114 at exposed surfaces 118 ofGe-containing layers 110 and form passivation layer 124 at exposedsurfaces 118. Passivation layer 124 may include portions of barrierlayer 114 that are not removed by plasma 122 at plasma step 120.

In certain embodiments, plasma 122 is generated from gases that includefluorine gas, nitrogen gas, and hydrogen gas. As examples, the fluorinegas used to generate plasma 122 may include NF₃, sulfur hexafluoride(SF₆), or carbon tetrafluoride (CF₄). Furthermore, although fluorine isdescribed, other halogens may be used as etchants.

As a particular example, the gases used to generate plasma 122 mayinclude a suitable combination of NF₃, N₂, and H₂. As another particularexample, the gases may include NF₃, ammonia (NH₃), and N₂. In certainembodiments, the N₂ could be replaced by a noble gas, such as Argon (Ar)or Krypton (Kr), or such a noble gas may be used in combination with N₂.As particular examples, gases/gas combinations used to generate plasma122 may include N₂/H₂/NF₃, N₂/NH₃/NF₃, Ar/NH₃/NF₃, N₂/H₂/Ar/NF₃, orN₂/H₂/NH₃/NF₃.

In an example in which gases used to generate plasma 122 include NF₃ andH₂, the ratio of NF₃ to H₂ may be an appropriate consideration. Thesuitable ratio (or range of ratios) may depend on a variety of factors,including other plasma process parameters and the concentration of Ge inGe-containing layers 110. Example ranges for the ratio of NF₃ to H₂ mayinclude from NF₃:H₂=1:1.2 to NF₃:H₂=1:3. Example ranges for the ratio ofNF₃ to N₂ may include from NF₃:N₂=1:2 to NF₃:H₂=1:8.

Other plasma process parameters for generating plasma 122 include gasflow rates, pressure, plasma source power, plasma bias power, time, andtemperature. The gases for forming plasma 122 may be provided at anysuitable flow rate. In various embodiments, the gas flow rates areNF₃=20 standard cubic centimeters per minute (sccm)-50 sccm, H₂=40sccm-100 sccm, N₂=50 sccm-300 sccm. In certain embodiments, plasma step120 may be performed at moderate pressure (e.g., about 100 mTorr toabout 500 mTorr, and in one example about 350 mTorr) and at moderatesource power (e.g., about 50 W to about 500 W, and in one example about200 W) without any bias power. Exposure time for plasma step 120 may beany suitable time. In certain embodiments, exposure time could be aslittle as about five seconds or less. In certain embodiments, exposuretime is about fifteen seconds. In certain embodiments, plasma step 120is performed at a temperature on substrate 102 of approximately −20° C.to approximately 40° C., and in one example at about ° C. It should beunderstood that particular values and ranges provided herein are forexample purposes only.

An example parameter set for plasma step 120 includes: pressure 350mtorr; source power (inductively-coupled plasma) 200 W; bias power 0 W;wafer processing temperature ° C.; and NF₃, H₂, and N₂ flow rates of 40sccm, 80 sccm, and 250 sccm, respectively.

As shown in FIG. 1C, plasma chamber 123 may be purged, which maysubstantially remove previous gases and associated agents (e.g.,fluorine agents 126, hydrogen agents 128, and nitrogen agents 130) fromplasma chamber 123 to reduce or eliminate interference by those gases oragents with subsequent plasma steps in plasma chamber 123.

FIGS. 1D-1E illustrate plasma steps 132 a and 132 b of process 100,referred to collectively as plasma step 132. In certain embodiments,plasma step 132 is an isotropic etch process. As illustrated in FIG. 1D,in plasma step 132 a, substrate 102 is exposed to plasma 134 to etchportions of film stack 104. For example, substrate 102 may be exposed toplasma 134 to form indents 136, or recesses, in film stack 104, withopposing end portions of Si layers 108 being removed relative toadjacent Ge-containing layers 110. During exposure of substrate 102 toplasma 134, passivation layer 124 on exposed surfaces 118 ofGe-containing layers 110 (and, in this example, on base layer 106)inhibits etching of Ge-containing layers 110 (and of base layer 106). Inother words, plasma 134 selectively etches Si layers 108 due at least inpart to the presence of passivation layer 124 on exposed surfaces 118 ofGe-containing layers 110 (and on exposed surfaces of base layer 106).

Plasma 134 includes etching agents and passivation agents. In certainembodiments, the etching agents of plasma include fluorine agents 126and the passivation agents include oxygen agents 140. For example,fluorine agents 126 may include atomic fluorine and oxygen agents 140may include atomic oxygen. Plasma 134 may be generated from gases thatinclude an etchant-containing gas (e.g., a fluorine-containing gas), apassivation-triggering gas (e.g., an oxygen-containing gas), and, incertain embodiments, a carrier gas (e.g. N₂). For example, plasma 134may be generated from gases including NF_(X), O_(Y), and N₂, and in aparticular example may be generated from gases including NF₃, O₂ (orcarbon dioxide (CO₂)), and N₂. In general, fluorine agents 126 act asthe etchant for etching Si layers 108. Although fluorine is described,other etchants may be used, if appropriate. As particular examples,gases/gas combinations used to generate plasma 134 may include, with N₂acting as an inert carrier gas: NF₃/O₂/N₂, SF₆/O₂/N₂, CF₄/O₂/N₂,NF₃/SF₆/O₂/N₂, NF₃/CF₄/O₂/N₂, or SF₆/CF₄/O₂/N₂.

As plasma 134 selectively etches Si layers 108, additional surfaces 138of Ge-containing layers 110 are exposed, and plasma 134 may formpassivation layer 124 on additional surfaces 138. For example,passivation layer 124 formed on additional surfaces 138 is an oxidepassivation layer resulting from the oxygen (e.g., oxygen agents 140) inplasma 134. In other words, passivation layer 124 is further formed overnewly exposed surfaces (e.g., additional surfaces 138) of Ge-containinglayers 110 as Si layers 108 are etched above, below, and/or betweenGe-containing layers 110. Passivation layer 124 at additional surfaces138 inhibits etching of Ge-containing layers 110 at additional surfaces138, while passivation layer 124 at exposed surfaces 118 ofGe-containing layers 110 continues to inhibit etching of Ge-containinglayers 110 at exposed surfaces 118. In certain embodiments, portions ofpassivation layer 124 include a Ge nitride or oxynitride passivationlayer (e.g., at exposed surfaces 118) and portions of passivation layer124 include an oxide passivation layer (e.g., at additional surfaces138).

Other process parameters for generating plasma 134 include gas flowrates, pressure, plasma source power, plasma bias power, time, andtemperature. The gases for forming plasma 134 may be provided at anysuitable flow rate. In certain embodiments, the etchant source gas flowrate is NF₃=50 sccm-300 sccm and O₂=75 sccm-450 sccm, and the NF₃:O₂ratio is an appropriate consideration and is in the range of 1:1.27 to1:3 sccm. In certain embodiments, plasma step 132 a may be performed atintermediate pressure of about 100 mTorr to about 500 mTorr, and in oneexample about 350 mTorr, and at intermediate source power of about 100 Wto about 400 W, and in one example about 200 W). Exposure time forplasma step 132 a may be any suitable time. In certain embodiments,exposure time could be as little as about five seconds or less. Incertain embodiments, exposure time is about fifteen seconds. In certainembodiments, plasma step 132 a is performed at a temperature onsubstrate 102 of approximately −20° C. to approximately 40° C., and inone example at about 0° C. It should be understood that particularvalues and ranges provided herein are for example purposes only.

An example parameter set for plasma step 132 a includes: pressure 350mtorr; source power (inductively coupled plasma) 200 W; bias power 0 W;wafer processing temperature 0° C.; and NF₃, O₂, and N₂ flow rates of200 sccm, 300 sccm, and 500 sccm, respectively.

As illustrated in FIG. 1E, in plasma step 132 b, injection (flow) of theetchant-containing gas (e.g., a fluorine-containing gas) into plasmachamber 123 is terminated, while the injection (flow) of apassivation-triggering gas (e.g., an O₂-containing gas) into plasmachamber 123 continues, resulting in generation of plasma 134′. Asdescribed above, etchants (e.g., fluorine agents 126) in plasma 134′have a longer lifetime than passivation reactants (e.g., oxygen agents140). Terminating injection/flow of the etchant-containing gas (e.g., afluorine-containing gas) into plasma chamber 123 while continuing toinject/flow the passivation-triggering gas (e.g., oxygen-containing gas)into plasma chamber 123, allows passivation layer 124 to continue to beformed on exposed surfaces 118 and additional surfaces 138 ofGe-containing layers 110 as the etchants (e.g., fluorine agents 126) inplasma 134′ expire, even as additional surfaces 138 continue to beexposed due to the presence of the expiring etchants. That is, duringexposure of substrate 102 to plasma 134′, passivation layer 124 onexposed surfaces 118 and additional surfaces 138 of Ge-containing layers110 (and, in the illustrated example, on base layer 106) inhibitsetching of Ge-containing layers 110 (and, in the illustrated example, ofbase layer 106) as the etchants (e.g., fluorine agents 126) in plasma134′ expire.

Plasma 134 in plasma step 132 b is labeled as plasma 134′ to representthat the content of plasma 134′ is changing following the termination ofinjecting the etchant-containing gas (e.g., NF₃) and the ongoingexpiration of etchants (e.g., fluorine atoms) in plasma 134′ duringplasma step 132 b. In certain embodiments, the process conditions forgenerating plasma 134′ are similar to those described above forgenerating plasma 134, except that the injection of theetchant-containing gas (e.g., fluorine-containing gas) is terminated.This disclosure contemplates other process conditions being used, ifappropriate.

This disclosure contemplates plasma step 132 b being executed for anysuitable time period, as it is believed that any additional time duringwhich passivation layer 124 is generated will improve the resultingprofile of the structure formed by process 100. Increased time periodsmay result in increasingly improved etch profiles; however, longer timeperiods also impact total processing time for forming a device usingsubstrate 102 as part of an integrated process. It has been found that 5to 30 seconds significantly improves an etch profile of film stack 104,though this disclosure is not limited to this range of time periods.

FIG. 1F illustrates substrate 102 following plasma steps 120 and 132. Inthe state illustrated in FIG. 1F, film stack 104 includes indents 136(of which two examples are labeled). Furthermore, due to the formationof indents 136, exposed ends 141 (of which one example is labeled) ofGe-containing layers 110 may be formed.

FIG. 1F shows certain measurements of resulting substrate 102, such asexposed end separation 142 and etched width 144. For example, exposedend separation 142 shows the remaining width (per this cross-section) ofSi layers 108 by measuring each Si layer 108 from a first exposedsurface 116 on a first side of film stack 104 to an opposing secondexposed surface 116 on a second side of film stack 104. Exposed endseparation 142 may be less than 20 nm in certain embodiments, andbetween 2 nm and 20 nm in a particular embodiment. The exposed endseparation may also refer to the separation of exposed ends prior toetching. Etched width 144 may measure how much of a particular Si layer108 was removed from an end of the particular Si layer 108. In otherwords, etched width 144 may measure the amount of an indent 136 of a Silayer 108. In certain embodiments, etched width 144 is about 5 nm toabout 15 nm. However, exposed end separation 142 and etched width 144may be outside these ranges depending on a given application.

Subsequent processing may be performed on substrate 102. For example,plasma steps 120 and 132 may be integrated into a process for formingGe-containing layers 110 into respective nanowires for a channel regionof a semiconductor device (e.g., a GAA device). In such a device,subsequent processing may include filling indents 136 with an insulator,removing remaining portions of Si layers 108, providing a gate oxidearound Ge-containing layers 110, and other associated steps, all ofwhich are provided for example purposes only. In such a device, exposedends 141 of Ge-containing layers 110 may serve as conductive contacts toa channel region formed in the area of film stack 104.

In the example of FIGS. 1A-1F, plasma steps 120 and 132 are performed inthe same plasma chamber of the same plasma tool, without removingsubstrate 102 from the plasma tool. Plasma steps 120 and 132, however,may be performed in the same or different plasma tools, as appropriate.Furthermore, although process 100 is described as including a purge step(e.g., as shown in FIG. 1C), the purge step may be omitted, ifappropriate.

Process 100 may provide one or more technical advantages. In certainembodiments, the surface modification step of process 100 (plasma step120) modifies barrier layer 114 at exposed surfaces 116 of Si layers 108and exposed surfaces 118 of Ge-containing layers 110, which may resultin improved surface roughness and local uniformity in film stack 104after a subsequent etch step to form indents 136 in film stack 104. Forexample, an edge profile of layers bordering indent 136 show improvedsurface roughness and local uniformity relative to conventionaltechniques, such as those that do not remove an NOL prior to performingan indent etch of a film stack like film stack 104 and instead simplyperform a single plasma etch (e.g., using fluorine- andoxygen-containing plasma, such as NF₃ and O₂) to remove the NOL and etchindents in the film stack. Improving surface roughness and localuniformity may improve channel mobility in resulting devices formedusing the substrate 102 of FIG. 1F.

In certain embodiments, despite removing or otherwise modifying barrierlayer 114, in contrast to conventional techniques that simply remove abarrier layer (e.g., an NOL) from the entire film stack (using a wet ordry etch) at the expense of desired selectivity in a subsequent step foretching indents in the film stack, the surface modification step ofprocess 100 forms passivation layer 124 on exposed surfaces 118 ofGe-containing layers 110. Passivation layer 124 inhibits etching ofexposed surfaces 118 of Ge-containing layers 110 in a subsequent etchprocess for etching indents in film stack 104, thereby providing desiredselectivity to the Ge-containing layers 110 in the subsequent etchprocess. Furthermore, the surface modification step and the subsequentetch process may be performed in a same plasma chamber of a plasma tool,making process 100 in situ in certain embodiments.

This disclosure contemplates performing process 100 with or withoutplasma step 132 b. Performing plasma step 132 b may provide one or moretechnical advantages. Terminating injection (flow) of theetchant-containing gas (e.g., NF₃) into plasma chamber 123 prior toterminating injection (flow) of the passivation-triggering gas (e.g.,O₂) into plasma chamber 123 allows the passivation reactants (e.g.,oxygen atoms) in the generated plasma (plasma 134′) to continue to formpassivation layer 124 on exposed surfaces 118 and additional surfaces138 of Ge-containing layers 110 while the etchants (e.g., fluorineatoms) in plasma 134′ expire. Depending on the time period used, thistechnique reduces or eliminates the opportunity for exposed surfaces 118and additional surfaces 138 of Ge-containing layers 110 to be exposed tothe etchants (e.g., fluorine atoms) in plasma 134′ without a passivationlayer 124 being formed on those exposed surfaces 118 and additionalsurfaces 138.

The continued formation of passivation layer 124 preserves theselectivity of plasma 134/134′ to etch Si layers 108 rather thanGe-containing layers 110, as the etchants (e.g., fluorine atoms) inplasma 134′ expire. Additionally, this preserved selectivity may reduceor eliminate “gouging” of Ge-containing layers 110 at the etch front inareas near the interface of Ge-containing layers 110 and their adjacentSi layers 108 in film stack 104, as well as excessive etching of Silayers 108. For example, the continued formation of passivation layer124 on additional surfaces 138 of Ge-containing layers 110, particularlyas those additional surfaces 138 continue to be revealed aftertermination of the flow of the etchant gas but prior to the expirationof the etchants in plasma 134′, may reduce or eliminate etching ofGe-containing layers 110 at the etch front. Furthermore, preserving theselectivity of plasma 134′ to etch Si and reducing or eliminating thisgouging of the Ge-containing layers at the etch front improves the localuniformity of Ge-containing layers 110 by providing a more uniformthickness of Ge-containing layers 110.

FIGS. 2A-2C illustrate cross-sectional views of an example substrate 102during example process 200 for processing substrate 102, according tocertain embodiments. Process 200 illustrates that plasma step 132 b ofFIG. 1E can be performed without performing plasma step 120. Process 200may begin with a substrate 102 similar to that illustrated and describedwith reference to FIG. 1A, the details of which are not repeated but areincorporated by reference.

As illustrated in FIG. 2A, barrier layer 114 is etched to remove some orall of barrier layer 114 from exposed surfaces 116 of Si layers 108,from exposed surfaces 118 of Ge-containing layers 110, and from exposedsurfaces of base layer 106 and hard mask 112. Barrier layer 114 may beremoved using any suitable process, including any suitable wet etch ordry etch process. In certain embodiments, substrate 102 is processedusing a dilute HF acid or a chemical oxide removal process to removesome or all of barrier layer 114. The process used to etch barrier layer114 might or might not remove some or all of hard mask 112, but forpurposes of this example, hard mask 112 is shown not to be removed orotherwise etched. Removing barrier layer 114 prior to performing theetch process described below with reference to FIG. 2B is optional.

FIGS. 2B-2C illustrate analogous plasma steps 132 a and 132 b to plasmasteps 132 a and 132 b, respectively, of FIGS. 1B and 1D. Therefore, thedetails are not repeated but are incorporated by reference. Becauseplasma step 120 is omitted in process 200, an entirety of passivationlayer 124 (both on exposed surfaces 118 and additional surfaces 138 ofGe-containing layers 110) may be an oxide passivation layer.

Process 200, following FIG. 2C, may produce a similar structure tosubstrate 102 illustrated in FIG. 1F. Thus, the details of FIG. 1F arenot repeated by are incorporated by reference. It will be understoodthat performing the barrier layer removal process of FIG. 2A rather thanplasma step 120 of FIG. 1B may result in some differences in theresulting substrate 102.

FIG. 3 illustrates an example gas flow timeline 300, according tocertain embodiments. In particular, gas flow timeline 300 shows theinitiation and termination of gas flows for an example of process 100. Agas flow also may be referred to as a gas injection. For purposes ofthis example, it will be assumed that the gases used to form plasma 122include NF₃, H₂, and N₂, and that the gases used to form plasma 134include NF₃, O₂, and N₂. Other gases may be used, as described elsewherein this disclosure. Although gas flows are shown as being initiated atthe same instant at the beginning of a time period (abbreviated TP 1, TP2, etc. in FIG. 3 ), gas flow initiation times may vary, if appropriate.Furthermore, although gas flows are shown as terminating at the end of atime period (if applicable), gas flows may be terminated prior to theend of a time period, if appropriate.

In the illustrated example, gas flow 302 corresponds to NF₃ gas, gasflow 304 corresponds to N₂, gas flow 306 corresponds to H₂ gas, and gasflow 308 corresponds to O₂ gas. Gas flows 302, 304, and 306 are activeduring time period TP 1, which corresponds to plasma step 120, tofacilitate generation of plasma 122. Gas flows 302, 304, and 306 areterminated at the end of time period TP 1/start of time period TP 2, andplasma chamber 123 is purged during time period TP 2. Gas flows 302,304, and 308 are active during time period TP 3, which corresponds toplasma step 132 a, to facilitate generation of plasma 134. Gas flow 302is terminated at the end of time period TP 3/start of time period TP 4,while gas flows 304 and 308 continue as active through time period TP 4to facilitate generation of plasma 134′. Time period TP 4 corresponds toplasma step 132 b.

FIG. 4 illustrates an example method 400 for processing a substrate 102,according to certain embodiments. Method 400 begins at step 402. At step404, substrate 102 is received. Substrate 102 may include film stack104, which may include alternating Si layers 108 and Ge-containinglayers 110 (e.g., as illustrated in FIG. 1A).

At step 406, in a first plasma step (e.g., plasma step 120), exposedsurfaces of substrate 102 (e.g., including exposed surfaces 116 of Silayers 108 and exposed surfaces 118 of Ge-containing layers 110) aremodified by exposing the exposed surfaces to plasma 122. In certainembodiments, plasma 122 includes fluorine, nitrogen, and hydrogen.Modifying the exposed surfaces may include, at step 406 a, removing orchemically modifying at least a portion of a barrier layer 114 fromexposed surfaces 116 of Si layers 108 and forming, at step 406 b,passivation layer 124 on exposed surfaces 118. Forming passivation layer124 may include removing barrier layer 114 from exposed surfaces 118 ofGe-containing layers 110 and modifying resulting exposed surfaces 118 ofGe-containing layers 110 to include passivation layer 124. In certainembodiments, plasma chamber 123 is purged between steps 406 and 408.

At step 408, in a second plasma step (e.g., plasma step 132), Si layers108 are etched using plasma 134 to form indents 136 in film stack 104 atSi layers 108 relative to adjacent Ge-containing layers 110. Passivationlayer 124 inhibits etching of Ge-containing layers 110. In certainembodiments, plasma 134 includes fluorine and oxygen. As a particularexample, the second plasma may be generated from gases including NF₃,O₂, and, in certain embodiments, N₂. In certain embodiments, step 408 isan isotropic etch process.

In certain embodiments, step 408 includes steps 408 a and 408 b. At step408 a, plasma 134 is generated. Generating plasma 134 includes injectingan etchant-containing gas (e.g., a fluorine-containing gas, such asNF₃), a passivation-triggering gas (e.g., O₂), and, in certainembodiments, a carrier gas (e.g., N₂) into plasma chamber 123, andplasma 134 includes etchant agents (e.g., fluorine agents 126) andpassivation agents (e.g., oxygen agents 140). At step 408 b, after atime period, injecting of the etchant-containing gas (e.g., afluorine-containing gas, such as NF₃) is terminated and injecting of thepassivation-triggering gas (e.g., O₂), and in certain embodiments thecarrier gas (e.g., N₂), is continued for an additional time period.

At step 410, additional fabrication steps are executed. The discussionof potential additional processing steps described above with referenceto FIG. 1F is incorporated by reference. For example, in certainembodiments, steps 406 and 408 are integrated into a process for formingGe-containing layers 110 into respective nanowires for a channel regionof a semiconductor device, such as a GAA device. At step 412, method 400ends.

FIG. 5 illustrates an example method 500 for processing a substrate 102,according to certain embodiments. Method 500 begins at step 502. At step504, substrate 102 is positioned in plasma chamber 123. Substrate 102may include film stack 104 that includes Si layers 108 and Ge-containinglayers 110 in an alternating stacked arrangement (e.g., as shown in FIG.1A).

At step 506, in a first plasma step (e.g., plasma step 120) executed inplasma chamber 123, substrate 102 (including film stack 104) is exposedto plasma 122. Plasma 122 may be generated from gases that includefluorine gas, hydrogen gas, and nitrogen gas. In certain embodiments,the gases include NF₃, N₂, and H₂.

At step 508, a purge of plasma chamber 123 is executed.

At step 510, in a second plasma step (e.g., plasma step 132 a) executedin plasma chamber 123, semiconductor substrate 102 (including film stack104) is exposed to plasma 134. Plasma 134 may be generated from gasesthat include fluorine gas (e.g., NF₃), oxygen gas (e.g., O₂), and, incertain embodiments, a carrier gas (e.g. N₂). Plasma 134 selectivelyetches Si layers 108. Step 510 may be an isotropic etch process. Incertain embodiments, step 510 includes generating plasma 134. Generatingplasma 134 includes injecting an etchant-containing gas (e.g., afluorine-containing gas, such as NF₃), a passivation-triggering gas(e.g., O₂), and, in certain embodiments, a carrier gas (e.g. N₂) intoplasma chamber 123, and plasma 134 includes etchant agents (e.g.,fluorine agents 126) and passivation agents (e.g., oxygen agents 140).At step 512, in the second plasma step (e.g., plasma step 132 b)executed in plasma chamber 123, after a time period, injecting of anetchant-containing gas into plasma chamber 123 is terminated andinjecting of a passivation-triggering gas and, in certain embodiments, acarrier gas into plasma chamber 123 is continued.

In certain embodiments, steps 506, 510, and 512 are executed in plasmachamber 123 without removing substrate 102 from plasma chamber 123(e.g., in situ).

At step 514, additional fabrication steps are executed. The discussionof potential additional processing steps described above with referenceto step 410 of FIG. 4 is incorporated by reference. At step 516, method500 ends.

FIG. 6 illustrates an example method 600 for processing a substrate 102,according to certain embodiments. Method 600 begins at step 602. At step604, substrate 102 is positioned in plasma chamber 123. Substrate 102may include film stack 104 that includes Si layers 108 and Ge-containinglayers 110 in an alternating stacked arrangement (e.g., as shown in FIG.1A). In certain embodiments, substrate 102 includes a barrier layer 114over film stack 104 (including, potentially, hard mask 112) and, in thisexample, base layer 106, and method 600 may include etching barrierlayer 114 (e.g., to remove barrier layer 114). Barrier layer 114 may beremoved using any suitable process, including any suitable wet etch ordry etch process, as described above. In certain embodiments, substrate102 of step 604 could be substrate following plasma step 132 a of FIG.1D.

At step 606, plasma 134 is generated in plasma chamber 123. Generatingplasma 134 includes injecting, into plasma chamber 123, gases includingan etchant-containing gas (e.g., a fluorine-containing gas, such asNF₃), a passivation-triggering gas (e.g., O₂), and, in certainembodiments, a carrier gas (e.g. N₂). Plasma 134 may include etchantagents (e.g., fluorine agents 126) and passivation agents (e.g., oxygenagents 140). At step 608, substrate 102, including film stack 104, isexposed to plasma 134 in plasma chamber 123.

At step 610, after a time period, injection of the etchant-containinggas into plasma chamber 123 is terminated while injection of theoxygen-containing gas and, in certain embodiments, the carrier gas intoplasma chamber 123 continues, generating plasma 134′. Terminatinginjection of the etchant-containing gas while continuing injection ofthe passivation-triggering gas and, in certain embodiments, the carriergas continues to form passivation layer 124 on exposed surfaces 118 andadditional surfaces 138 of Ge-containing layers 110, thereby protectingGe-containing layers 110 from etching as etching agents (e.g., fluorineagents 126) expire in plasma chamber 123. In certain embodiments, afterterminating injecting the etchant-containing gas into plasma chamber123, injecting the oxygen-containing gas and, in certain embodiments,the carrier gas into plasma chamber 123 continues for greater than aboutone second and possibly greater than about five seconds.

At step 612, additional fabrication steps are executed. The discussionof potential additional processing steps described above with referenceto step 410 of FIG. 4 is incorporated by reference. At step 614, method600 ends.

FIG. 7 illustrates an example device 700 including a substrate with arecessed alternating film stack according to certain embodiments. Atleast a portion of device 700 may be formed using any of the processesand methods as described herein.

Device 700 includes a substrate 702 that includes a channel material 704(e.g., Ge or SiGe) and a gate material 706, (e.g. SiGe or Si). Channelmaterial 704 may correspond to Ge-containing layers 110 of substrate102, at some point after process 100 or process 200. Device 700 may be aGAA device as shown here or may be any other device, such as a finfield-effect transistor (FinFET). Device 700 also may include isolationregions 708. In certain embodiments, isolation regions 708 are shallowtrench isolations (STIs).

Device 700 may be fabricated by first forming a recessed alternatingfilm stack 710 (which may correspond to film stack 104 following process100 or process 200, possibly with additional subsequent processes) andthen depositing additional gate material 706 over recessed alternatingfilm stack 710. Specifically, device 700 may be formed byheteroepitaxial growth of alternating Si and Ge or SiGe layers which arethen patterned and recessed vertically to expose the Ge or SiGe layerslaterally.

The application of embodiments described herein may advantageously be anoptimal solution for the 5 nm node, 3 nm node, or lower. For example,the GAA device architecture may be suitable for scaling beyond the 7 nmnode. The GAA device architecture may address short channel effectsfound in some FinFET architectures by wrapping the gate around theentire channel instead of only three sides. This could reduce oreliminate current leakage occurring under the gate of the FinFET,therefore reducing non-active power losses.

FIG. 8 illustrates a block diagram of an example plasma tool 800,according to certain embodiments. Although a particular plasma tool 800is illustrated and described, any suitable type of plasma tool may beused. Plasma tool 800 may be used to execute plasma steps 120, 132 a,and/or 132 b, for example.

Plasma tool 800 includes plasma chamber 123 in which a semiconductorsubstrate (e.g., substrate 102) is processed using a plasma (e.g.,plasma 122, 134, and/or 134′). Plasma chamber 123 includes a substratetable 802 configured to support substrate 102 during processing. Forexample, substrate 102 may be positioned on substrate table 802 in thecondition shown in FIG. 1A or 2A. Exposed surfaces 116 of Si layers 108and exposed surfaces 118/additional surfaces 138 of Ge-containing layersare modified within plasma chamber 123 by injecting a plasma (e.g.,plasma 122) through a shower head 804. Si layers 108 are selectivelyetched within plasma chamber 123 by injecting plasma (e.g., plasma134/134′) through shower head 804. Shower head 804 may include a singlemixed reaction cavity that is filled with precursor gases, mixing gases,and carrier gases that mix to form plasma 122/134/134′ and a set of exitholes for dispensing plasma 122/134/134′ toward substrate 102.

Plasma chamber 123 includes and/or is otherwise coupled to a vacuum pump806 coupled to a vacuum line 808 to purge residual gases from plasmachamber 123 and also may include and/or otherwise be coupled to apressure system to maintain a target pressure in certain embodiments.Plasma chamber 123 may further include machine tools such as a heater810 and temperature sensor 812 used to heat substrate 102 and controlthe temperature within plasma chamber 123 and/or of substrate 102.

Shower head 804 may be coupled to a precursor gas line 814, a mixturegas line 816, and a carrier gas line 818, through which gases injectedinto plasma chamber 123 to generate plasmas 122, 134, and 134′ are fed.Plasma tool 800 may include a system of flow controllers and sensors forcontrolling gas flow (e.g., mass flow rate). For example, plasma tool800 may include a first flow controller 820, a second flow controller822, a third flow controller 824, vacuum pump 806, heater 810,temperature sensor 812, voltage-current (V-I) sensor 826, and substratesensors 828, 830, 832, and 834 (828-834). Precursor gas line 814,mixture gas line 816, and carrier gas line 818 are coupled to andcontrolled by first, second, and third flow controllers 820, 822, and824, respectively.

Plasma tool 800 may include a controller 836 to control various aspectsof plasma steps 120, 132 a, and/or 132 b. Controller 836 may beimplemented in any suitable manner. For example, controller 836 may beor include a computer or one or more programmable ICs programmed toprovide functionality described herein. In a particular example, one ormore processors (e.g., microprocessor, microcontroller, centralprocessing unit, etc.), programmable logic devices (e.g., complexprogrammable logic device), field programmable gate array, etc.), and/orother programmable ICs are programmed with software or other programminginstructions to implement functionality described herein for controller836. The software or other programming instructions can be stored in oneor more non-transitory computer-readable mediums, and, when executed bythe programmable ICs, cause the programmable ICs to perform operationsdescribed herein.

Machine components such as heater 810 and temperature sensor 812 ofplasma chamber 123, as well as flow controllers 820, 822, and 824,vacuum pump 806, and other components external to plasma chamber 123,are coupled to and controlled by controller 836.

Equipment sensors measure equipment parameters, such as substrate table802 temperature, heater 810 currents, and vacuum pump 806 speed andtemperature, and provide signals to ensure the equipment is operatingproperly. Process sensors measure process parameters, such as processtemperature, process pressure, plasma density, gas flow rates, and gascomposition, and provide signals to ensure the process is operatingproperly. The data from the equipment and process sensors providefeedback to controller 836 continuously throughout plasma steps 120, 132a, and/or 132 b. Controller 836 can make adjustments in real time tokeep the equipment and process close to center of specifications.

Controller 836 receives data from the sensor(s) and controls one or moreprocess parameters of plasma chamber 123 based on the sensor data.Controller 836 may analyze the data collected by the sensor(s),determine when to modify or end one or more steps of plasma steps 120,132 a, and/or 132 b, and provide feedback to control process parametersof components of plasma chamber 123.

Controller 836 may be connected to V-I sensor 826, and substrate sensors828-834 to monitor plasmas 122/134/134′ as substrate 102 is exposed tothese plasmas to provide plasma conditions as well as optionallycomposition and thickness data in real time. Controller 836 may use thisfeedback data to continuously adjust plasma steps 120, 132 a, and/or 132b as substrate 102 is selectively etched using plasmas 122/134/134′ and,for example, to turn off plasma steps 120, 132 a, and/or 132 b when thetarget indent (e.g., etched width 144) is reached. Specifically,controller 836 may receive measurement data from substrate sensors828-834, and temperature sensor 812 and controller 836 may send controlsignals to first, second, and third flow controllers 820, 822, and 824,and to vacuum pump 806 and heater 810.

Controller 836 may receive measurement or metrology data from substratesensors 828-834 taken at multiple points across substrate 102 to measureprocess uniformity and the thickness and composition of passivationlayer 124 (formed from exposure of substrate 102 to plasmas122/134/134′), exposed end separation 142, and/or the target indent(e.g., etched width 144) in situ and in real time. For example, multipleacross-substrate sensors in a multi-substrate plasma tool can be used tomonitor and tune these properties of substrate 102, both from the top tothe bottom of substrate 102 and from center to the edge of substrate102.

Substrate sensors 828-834 may be coupled to and/or located within plasmachamber 123 for monitoring parameters of substrate 102, plasma tool 800and/or plasma steps 120, 132 a, and/or 132 b. Substrate sensors 828-834may include optical sensors (e.g., cameras, lasers, light,reflectometer, spectrometers, ellipsometric, etc.), capacitive sensors,ultrasonic sensors, gas sensors, or other sensors. For example, one ormore optical sensors may measure in real time (during plasma steps 120,132 a, and/or 132 b) the thickness and refractive index of the materialat exposed surfaces 118/additional surfaces 138 of Ge-containing layers110 and surfaces of base layer 106 (e.g., where passivation layer 124 isbeing formed), exposed end separation 142, and/or an etched width 144(or another suitable measurement). As another example, a spectrometermay measure in real time (during plasma steps 120, 132 a, and/or 132 b)a film thickness of the material at these locations of semiconductordevice 102. In yet another embodiment, a residual gas analyzer (RGA) maydetect in real time (during plasma steps 120, 132 a, and/or 132 b)precursor breakdown for real-time chemical reaction completiondetection.

Controller 836 may receive user-input process parameters, including, forexample, etch rate, conformality, profile (of film stack 104), anddeposition rate (e.g., of passivation layer 124) based on plasmaparameters such as pressure, temperature, RF source power, RF biaspower, RF waveform (e.g., continuous wave RF, pulsed RF, square pulse,sawtooth pulse, etc.), etch time, and composition and flow rates ofgases, advantageously allowing a user to tune plasmas to meet a targetlocal critical dimension uniformity.

Based on data from substrate sensors 828-834 and user-specified processparameters, controller 836 generates control signals to temperaturesensor 812 and heater 810 to adjust the heat within plasma chamber 123and/or on substrate 102. As heater 810 heats plasma chamber 123,controller 836 constantly or periodically monitors temperature sensor812 to track the temperature of plasma chamber 123 and/or substrate 102to send control signals to heater 810 to maintain the temperature inplasma chamber 123 and/or on substrate 102. Once controller 836determines, based on data from temperature sensor 812, that the targettemperature of plasma chamber 123 and/or on substrate 102 has beenreached, controller 836 generates signals to activate first, second, andthird flow controllers 820, 822, and 824 and provide, based onuser-input process parameters, target flow rates to first, second, andthird flow controllers 820, 822, and 824. Once controller 836 determinesthat the corresponding flow rates are established, controller 836provides power to plasma chamber 123 to power plasma 122/134/134′through bias and source electrodes, which may be adjusted based onmeasurements from V-I sensor 826. First, second, and third flowcontrollers 820, 822, and 824 each may be a closed loop control systemconnected to a flow rate sensor and an adjustable proportional valvethat allows each flow controller to monitor and internally maintain thetarget flow rates of each gas via the flow rate sensor and theadjustable proportional valve. Once controller 836 determines, based onuser input, that the etch process time has been met, controller 836generates control signals to deactivate first, second, and third flowcontrollers 820, 822, and 824, which may be deactivated at the same ordifferent times, as appropriate.

Controller 836 may analyze substrate sensor data to determine when toend plasma steps 120, 132 a, and/or 132 b. For example, controller 836may receive data from an RGA to detect an endpoint of plasma steps 120,132 a, and/or 132 b. In another example, controller 836 may usespectroscopic ellipsometry to detect an average film thickness ofpassivation layer 124, exposed ends 141 of Ge-containing layers 110,and/or exposed end separation 142 during plasma steps 120, 132 a, and/or132 b and indicate changes during the plasma steps. In another example,controller 836 may use spectroscopic ellipsometry to detect therefractive index of the material at exposed surfaces 118/additionalsurfaces 138 of Ge-containing layers 110 and surfaces of base layer 106(e.g., where passivation layer 124 is being formed) during plasma steps120, 132 a, and/or 132 b and indicate film composition change during theplasma steps. Controller 836 may automatically end plasma steps 120, 132a, and/or 132 b when an exposed end separation 142 and/or an etchedwidth 144 (or another suitable measurement) objective is achieved.Controller 836 may automatically adjust one or more parameters such asthe gas ratios, for example, during plasma steps 120, 132 a, and/or 132b to achieve the desired etch profile of film stack 104. Controller 836and the data from substrate sensors 828-834 also may be used to achievea desired semiconductor substrate throughput.

Although described for a particular application of formingnanowires/nanosheets for GAA devices, this disclosure may be used in anytype of isotropic etch of Si that is selective to Ge-containing layers.Furthermore, although the etch being performed is primarily described asbeing for forming indents in film stack 104 by removing portions ofopposing ends of Si layers 108, processes 100 and 400 may be used toremove substantially all portions of Si layers 108, which may bereferred to as releasing Ge-containing layers 110.

Although this disclosure describes particular process/method steps asoccurring in a particular order, this disclosure contemplates theprocess steps occurring in any suitable order. While this disclosure hasbeen described with reference to illustrative embodiments, thisdescription is not intended to be construed in a limiting sense. Variousmodifications and combinations of the illustrative embodiments, as wellas other embodiments of the disclosure, will be apparent to personsskilled in the art upon reference to the description. It is thereforeintended that the appended claims encompass any such modifications orembodiments.

What is claimed is:
 1. A method of processing a semiconductor substrate,the method comprising: receiving a semiconductor substrate thatcomprises a film stack, the film stack comprising a firstgermanium-containing layer, a second germanium-containing layer, and afirst silicon layer positioned between the first germanium-containinglayer and the second germanium-containing layer; in a first plasma step,generating a first plasma and modifying exposed surfaces of the firstgermanium-containing layer, the second germanium-containing layer, andthe first silicon layer by exposing the exposed surfaces to the firstplasma, modifying the exposed surfaces including: removing at least aportion of a native oxide layer from the exposed surfaces of the firstsilicon layer; and forming a passivation layer on the exposed surfacesof the first germanium-containing layer and the secondgermanium-containing layer; and in a second plasma step, generating asecond plasma and etching, using the second plasma, the first siliconlayer to form an indent in the film stack at the first silicon layerbetween the first germanium-containing layer and the secondgermanium-containing layer, the passivation layer inhibiting etching ofthe first germanium-containing layer and the second germanium-containinglayer, the second plasma being different than the first plasma.
 2. Themethod of claim 1, wherein the first germanium-containing layer and thesecond germanium-containing layer are both germanium layers or are bothsilicon-germanium layers.
 3. The method of claim 1, wherein forming thepassivation layer comprises: removing at least a portion of a nativeoxide layer from the exposed surfaces of the first germanium-containinglayer and the second germanium-containing layer; and modifying resultingexposed surfaces of the first germanium-containing layer and the secondgermanium-containing layer.
 4. The method of claim 1, wherein the secondplasma comprises fluorine and oxygen.
 5. The method of claim 1, whereinthe second plasma is generated from gases comprising NF₃, O₂, and N₂. 6.The method of claim 1, wherein: the first plasma step further comprisesgenerating the first plasma from gases comprising fluorine-containinggas, hydrogen-containing gas, and nitrogen-containing gas, the firstplasma comprising fluorine, hydrogen, and nitrogen; and the secondplasma step further comprises generating the second plasma from gasescomprising fluorine-containing gas and oxygen-containing gas, the secondplasma comprising fluorine and oxygen.
 7. The method of claim 1,wherein: the film stack further comprises a third germanium-containinglayer and a second silicon layer positioned between the secondgermanium-containing layer and the third germanium-containing layer; andthe method comprises: in the first plasma step, modifying exposedsurfaces of the third germanium-containing layer and the second siliconlayer by exposing the exposed surfaces of the third germanium-containinglayer and the second silicon layer to the first plasma, modifying theexposed surfaces of the third germanium-containing layer and the secondsilicon layer including: removing at least a portion of a native oxidelayer from the exposed surfaces of the second silicon layer; and formingthe passivation layer on the exposed surfaces of the thirdgermanium-containing layer; and in the second plasma step, etching,using the second plasma, the second silicon layer to form an indent inthe film stack at the second silicon layer between the secondgermanium-containing layer and the third germanium-containing layer, thepassivation layer inhibiting etching of the third germanium-containinglayer.
 8. The method of claim 1, wherein the first plasma step and thesecond plasma step are integrated into a process for forming the firstgermanium-containing layer and the second germanium-containing layerinto respective nanowires for a channel region of a semiconductordevice.
 9. A method of processing a semiconductor substrate, the methodcomprising: receiving a semiconductor substrate that comprises a filmstack, the film stack comprising a first germanium-containing layer, asecond germanium-containing layer, and a first silicon layer positionedbetween the first germanium-containing layer and the secondgermanium-containing layer; in a first plasma step, modifying exposedsurfaces of the first germanium-containing layer, the secondgermanium-containing layer, and the first silicon layer by exposing theexposed surfaces to a first plasma, modifying the exposed surfacesincluding: removing at least a portion of a native oxide layer from theexposed surfaces of the first silicon layer; and forming a passivationlayer on the exposed surfaces of the first germanium-containing layerand the second germanium-containing layer; and in a second plasma step,etching, using a second plasma, the first silicon layer to form anindent in the film stack at the first silicon layer between the firstgermanium-containing layer and the second germanium-containing layer,the passivation layer inhibiting etching of the firstgermanium-containing layer and the second germanium-containing layer,wherein the second plasma step further comprises: generating the secondplasma, generating the second plasma comprising injectingfluorine-containing gas, oxygen-containing gas and a carrier gas into aplasma chamber of a plasma tool; and terminating, after a time period,injecting the fluorine-containing gas into the plasma chamber andcontinuing injecting the oxygen-containing gas and the carrier gas intothe plasma chamber.
 10. The method of claim 9, wherein the firstgermanium-containing layer and the second germanium-containing layer areboth germanium layers or are both silicon-germanium layers.
 11. Themethod of claim 9, wherein forming the passivation layer comprises:removing at least a portion of a native oxide layer from the exposedsurfaces of the first germanium-containing layer and the secondgermanium-containing layer; and modifying resulting exposed surfaces ofthe first germanium-containing layer and the second germanium-containinglayer.
 12. The method of claim 9, wherein the second plasma comprisesfluorine and oxygen.
 13. The method of claim 9, wherein the secondplasma is generated from gases comprising NF₃, O₂, and N₂.
 14. Themethod of claim 9, further comprising purging a plasma chamber of aplasma tool between the first plasma step and the second plasma step.15. The method of claim 9, wherein the first plasma step and the secondplasma step are integrated into a process for forming the firstgermanium-containing layer and the second germanium-containing layerinto respective nanowires for a channel region of a semiconductordevice.
 16. A method of processing a semiconductor substrate, the methodcomprising: receiving a semiconductor substrate that comprises a filmstack, the film stack comprising a first germanium-containing layer, asecond germanium-containing layer, and a first silicon layer positionedbetween the first germanium-containing layer and the secondgermanium-containing layer; in a first plasma step, modifying exposedsurfaces of the first germanium-containing layer, the secondgermanium-containing layer, and the first silicon layer by exposing theexposed surfaces to a first plasma, modifying the exposed surfacesincluding: removing at least a portion of a native oxide layer from theexposed surfaces of the first silicon layer; and forming a passivationlayer on the exposed surfaces of the first germanium-containing layerand the second germanium-containing layer; and in a second plasma step,etching, using a second plasma, the first silicon layer to form anindent in the film stack at the first silicon layer between the firstgermanium-containing layer and the second germanium-containing layer,the passivation layer inhibiting etching of the firstgermanium-containing layer and the second germanium-containing layer;wherein the method further comprises purging a plasma chamber of aplasma tool between the first plasma step and the second plasma step.17. The method of claim 16, wherein the first germanium-containing layerand the second germanium-containing layer are both germanium layers orare both silicon-germanium layers.
 18. The method of claim 16, whereinforming the passivation layer comprises: removing at least a portion ofa native oxide layer from the exposed surfaces of the firstgermanium-containing layer and the second germanium-containing layer;and modifying resulting exposed surfaces of the firstgermanium-containing layer and the second germanium-containing layer.19. The method of claim 16, wherein the second plasma comprises fluorineand oxygen.
 20. The method of claim 16, wherein the second plasma isgenerated from gases comprising NF₃, O₂, and N₂.